Ceramics
Ceramics are:
inorganic, nonmetallic, solids, crystalline,
amorphous (e.g. glass).
Hard, brittle, stable to high temperatures, less
dense than metals (up to 40%).
More elastic than metals (do not readily deform
under stress).
Very high melting (up to 2800oC).
Ceramics
Ceramics can be covalent-network and/or ionic
bonded.
Typical examples:
Aluminates
Carbides
Oxides
Silicates
alumina (Al2O3)
silicon carbide (SiC)
zirconia (ZrO2) and beryllia (BeO)
silica (SiO2)
Ceramics
Processing of Ceramics
Ceramics are very brittle and shatter when struck.
Bonding prevents atoms from sliding over one
another.
Compare steel and a clay pot!
Ceramics
Processing of Ceramics
Small defects (micro-cracks and voids) developed
during processing make ceramics weaker.
Micro-crack areas more susceptible to more stress.
Minimise stress fractures by using very pure uniform
particles (< 1m or 10-6m in diameter).
Ceramics
Processing of Ceramics
Sintering:
Heating of very pure uniform
particles (about 10-6 m in diameter)
under high temp & pressure to force
particles to bond.
During sintering the particles coalesce without
melting.
Alumina (Al2O3) melts at 2050oC but coalesces at
1650oC
Ceramics
Processing of Ceramics
Sol-gel process: formation of pure uniform particles.
Metal alkoxide is formed (e.g. Ti(OCH2CH3)4).
Sol formed by reacting metal alkoxides with water (to
form Ti(OH)4).
Ceramics
Processing of Ceramics
Ti(s) + 4CH3CH2OH(ℓ)  Ti(OCH2CH3)4(s) + H2(g)
Ti(OCH2CH3)4 + 4H2O(ℓ)  Ti(OH)4 + 4CH3CH2OH(ℓ)
Direct addition of water to Ti(s) leads to complex
mixtures of oxides and hydroxides.
Alkoxide intermediate ensures a uniform suspension of
Ti(OH)4. This is the sol stage.
Ceramics
Processing of Ceramics
Acidity or basicity of the sol is adjusted to split water
from between two of the Ti-OH bonds.
(HO)3Ti-O-H(s)
+
(HO)3Ti-O-Ti(OH)3(s)
H-O-Ti(OH)3(s)
+
H2O(l)
This is another example of a condensation reaction.
Ceramics
Processing of Ceramics
Condensations also occurs at some of the other OH
groups.
This produces a three-dimensional network
This is the Gel and the suspension of extremely
small particles has the consistency of gelatine.
Ceramics
Processing of Ceramics
Gel is heated carefully at 200oC to 500oC to remove
water and the gel is converted into finely divided
oxide powder.
Particle size range of 0.003 to 0.1m in diameter.
Ceramics
Applications of Ceramics
Used in cutting tool industry (alumina reinforced with
silicon carbide).
Used in electronic industry (semiconductor integrated
circuits usually made of alumina).
Ceramics
Applications of Ceramics
Piezoelectric materials : Quartz (crystalline SiO2)
Generates an electrical potential after mechanical
stress and is used in watches and ultrasonic
generators.
Heat resistant tiles: (Silica fibres reinforced with
aluminum borosilicate fibres)
Tiles (0.2 g.cm-3) on the space shuttle has surface
temperature of 1250oC while shuttle surface
temperature gets to 180oC during re-entry.
Ceramics
Superconductors show no resistance to flow of
electricity.
Superconducting behavior only starts below the
superconducting transition temperature, Tc.
Ceramics
Resistance
Graph of resistance against temperature for a
superconducting material
Transition temp. (Tc)
Temperature
Ceramics
Meissner effect: permanent magnets levitate over
superconductors. The superconductor excludes all
magnetic field lines from its volume, so the magnet
floats in space.